The value usually depends truly on the mileage. For no matter how much restoration you do, you cannot turn back the mileage. That being said I have restored a 1965 caddy eldorado, 66 caddy conv., and a 64 convert., spent anywhere from 3,000-15,000 depending on the level of restoration. And these are by no means "showroom" cars. Had a buddy spend 30,000 on a 64 caddy that was one step below a frame off resto. I always used mine as daily drivers, because I wanted to enjoy them everyday. If it is worth it to you than do it.